Protection system with manual reset means operable only on clearing of the fault

ABSTRACT

A protection system is provided for effecting disruption of the power being supplied to a load responsive to a sensed variation in the operation of a system being protected and for preventing re-application of power to the load until the fault dissipates. The system includes a first selectively operable switch means for connecting and disconnecting the load from a source of power and a switch control means operatively connected to the switch means for effecting opening and closing thereof so as to effect removal and re-application of power to the load, respectively responsive to energization and de-energization thereof. The switch control means remains in an energized condition subsequent to energization due to the establishment of a magnetic bias until neutralization of the magnetic bias is effected. A selectively energizable current switch is coupled to the switch control means for controlling its operation, while means responsive to the operation of the system being protected are coupled to the current switch for controlling conduction thereof responsive to predetermined variations in the operation of the system being protected. In addition, selectively operable means are coupled to the switch control means for effecting neutralization of the magnetic bias only when the current switch returns to a nonconductive condition.

Nov. 7, 1972 United States Patent Obenhaus PROTECTION SYSTEM WITH MANUALRESET MEANS OPERABLE ONLY ON CLEARING OF THE FAULT' [72] Inventor:Robert E. Obenhaus, South Easton,

, p Mass.

[73] Assignee: Texas Instruments Incorporated, Dallas, Tex.

includes a first selectively operable switch means for connecting anddisconnecting the load from a source [22] Filed: Sept. 30, 1971v [2]]Appl. No.: 185,103

of power and a switch control means operatively connected to the switchmeans for effecting opening and closing thereof so as to effect removaland re-application of power to the load, respectively responsive toenergization and de-energization thereof. The switch 317/33 317/41,317/52 317/54 control means remains in an energized condition sub- [51]Int. 5/04 sequent to energization due to the establishment of a [58]Field of Search ..3l7/54, 60, 52, 41,13 A, 13 C,

magnetic bias until neutralization of the magnetic bias 317/36 TD, 23,33 R, 18 D is effected. A selectively energizable current switch iscoupled to the switch control means for controlling its 56 I ReferencesCited UNITED STATES PATENTS operation, while means responsive to theoperation of the system being protected are coupled to the currentswitch for controlling conduction thereof responsive to predeterminedvariations in the operation of the system being protected. in addition,selectively operable means are coupled to the switch control means foreffecting neutralization of the magnetic bias only Primary Examiner-lMlllel' when the current switch returns to a non-conductive condition.

Assistant Examiner-Harvey Fendelman Attorney-Harold Levine et al.

16 Claims, 1 Drawing Figure The present invention relates-generally toprotection,

systems and rnoreparticularly is directed to, an improved electronicprotection system for' effecting disruption-of'the power to a load. inthe presence of a sensed fault in theoperation of the 2systembeingprotected and for maintaining such a condition. l

A wide variety of electronic protection. systems are currentlyavailableforsensing operation of a system and disrupting the powerbeingsuppliedthereto in the event of a sensed fault condition such, asover-current, over-heating, etc..Generally such protection systems aredesigned; to operate in an automatic fashion to effect'opening of a mainpower switch, orthe like, so as to cause the applied powerto bedisconnectedfrom the load in the event of a sensed fault condition.However, it is generally necessary that means be providedforeffectingre-application of power tothe load after such a power disruption hasbeen effected. This is usually accomplished by providing suitable resetswitch means or re-cycling the-power input to the load. For example, insuch systems a manualreset switchmay be provided for re-closing the mainpower switch to permit power to be reapplied to the load. However, suchprotection systems maybe quite dangerous in certain instances, Forexample, the fault condition: may remain in existence, while actuationof the manual reset switch may permit power to be reapplied to the load,thereby overriding the protection system, evenif onlytemporarily, whichmay result.in,damage tothe system beingprotected. Similarly, incertaininstances, resetting of-the system maybe effected prior todisconnecting the main power source from'thesystem thereby permitting apotentially dangeroussituationto occur with respect to personnel workingon the system. Furthermore, some systems may be r e-actuated upon there-application 'of power to the load externally so asto effect automaticoperation of the reset switch, which also, may lead to potentiallydangerous situations. It is thus extremely advantageous to provide asystem which effects complete power disruption and maintains thiscondition even when manual'reset is attempted unless the fault conditionhas dissipated. Such a system may be said to have a memory function inthat it maintains a particular state, i.e., a state of deenergi'zation,until proper conditions for the re-application of power have beenestablished. it is furthermore desirable to provide such a system inwhich the continued presence of a fault condition prevents continuedcycles of re-application and disruption of power to the load.

Accordingly, it is an object of the present invention to provide animproved protection system in which positive disruption of the powerbeing supplied to a load is effected and maintained responsive to asensed Thesole drawing is a schematic circuit diagram of a preferredembodiment of the present invention.

Referring generallygto the drawing, a protection system in accordancewith the present invention generally includes a first selectivelyoperable switch means 10 which is normally closed for supplying power tothe load 12 from a suitable power source 14. More particularly, thesystem includes a power supply networklfi for supplying power from: thepower source 14 to an output network 18, including the switch 10, whichis serially connected to. the load 12 for controlling the supply ofpower thereto. In addition, a fault sensingnetwork .20 is provided whichis responsive to sensed variations in the operation of the system beingprotected, such as a system,(not shown) coupled to the load 12. Thefault sensing network in turn is coupled to comprises a suitable a.c.power source suchas'a conventional 115 volt, 60 h; power source which isconnected to the power supply system 16 through a suitable main powerswitch 24 which upon closure effects applicationof power to the system.In this regard, upon application of power to the system it may beseenthat power is directly supplied to the load 12 from the power source14 since the selectively operable switch 10 is in a normally closedcondition in the absence of a sensed fault causing opening thereof.Power for energizing the fault sensing network 20 and the controlnetconnected to a current limiting resistor 28 and toa;

filter capacitor 30 todefine a half-wave unregulated dc. power sourceconnected across the a.c. power source 14. In order to provide a morestable source of operating power for. the fault sensing network 20 andfault condition.

p It is a further object of the present invention to pro vide animproved protection system in which the power the control network 22 acoupling resistor 32 is connected intermediate the resistor 28 and thecapacitor 3 0:while a voltage regulator device, such as a zener diode34, is connected across the resistor 32 and capacitor 30 to provide aregulated d.c. voltage source which supplies regulated dc. power to thefault sensing network 20 and the control network 22, thereby stabilizingcircuit operations substantially independently of variations in linevoltage due to transients, or the like. i

Fault sensing, i.e., sensing improper operation of the system beingprotected, is-accomplished by the fault sensing network 20 which may bearranged to sense various types of undesired operation, such asoverheating, over-current, etc. The fault sensing function is preferablyaccomplished by means of a voltage divider configuration 36 whichincludes first and second resistors 38 and 40 with a junction 42therebetween, while a variable impedance sensor 44 is connected to oneof the resistors of the voltage divider, and in the illustratedembodiment is connected to the resistor 40.

The variable impedance sensor 44 may comprise any one of a variety ofvariable impedance elements, and preferably comprises athermistor whichvaries in resistance in response to temperature. Accordingly, the sensormay be arranged thermally coupled to but electrically isolated from thesystem being protected. For example, the sensor 44 may be suitablythermally coupled to a heat responsive resistor (not shown) in the isystem being protected such that excessive current flow through the heatresponsive resistor could cause the temperature of the sensing element44 to similarly increase with a consequent variation in its resistanceso as to effect a variation'in the voltage level established at thevoltage divider junction 42 which, in turn, causes the fault sensingnetwork 20 to effect operation of the switch means 10.- Similarly, thevariable impedance sensor 44 could comprise a photo-resistor, aphoto-cell, etc., which could be optically coupled to the system beingprotected for sensing undesired variations in the operation thereof.However, in the illustrated embodiment the variable impedance sensingelement 44 preferably comprises a thermistor having a positivetemperature coefficient of resistance which is adapted to be thermallycoupled to a heating resistor, orthe like, in the system being protectedfor sensing undesired over-current operation thereof so as to effectopening of the switch means 10, thereby disrupting the power beingapplied to the load which also may be suitably coupled to the systembeing protected for effecting deenergization thereof. In addition, avoltage integrating capacitor 46 is connected across the resistor 40 ofthe voltage divider 36 in order to prevent the occurrence of transientsor spurious signals from inadvertently effecting operation of the faultsensing network. Thus, when the voltage divider configuration 36 whichis connected across the zener diode 34, is supplied with power of firstpreselected voltage level is established at the junction 42, when thevariable impedance sensor 44 is not exposed to improper operation of thesystem being protected. However, when improper operation of the systembeing protected occurs the impedance of the sensor 44 varies andaccordingly the voltage developed across the leg of the voltage divider36 including the resistor 40 varies so that a second preselected voltagelevel is established at the junction 42 as an indication of undesiredoperation of the system being protected. This second preselected voltagelevel may, in turn, be utilized for effecting opening of the selectivelyoperable switch 10, as will now-be explained in detail.

The establishmentof the second preselected voltage level at the junction42 effects energization of a first selectively energizable switchcontrol means 48, which is operatively connected to the switch andadapted to effect opening of the switch 10 in response to energizationof the switch control means 48. Furthermore, in accordance with animportant feature of the present invention, once energized the switchcontrol means 48 retains the switch 10 in an open condition due to theestablishment of a magnetic bias, as will be explained hereinafter,until neutralization of the magnetic bias is effected subsequent todissipation of the sensed fault condition. Energization of the firstselectively energizable switch control means 48 is accomplishedresponsive to conduction of a selectively energizable current switch 50which is coupled thereto, the conduction of the current switch 50 beingcontrolled responsive to the voltage level established at the junction42. Similarly, reverse action is accomplished by a selectively operablemeans 52 which is also coupled to the current switch 50 and may beenergized only when the current switch 50 is in a non-conductivecondition. More particularly, the current switch 50 includes a controlelement 52 which is coupled to the junction 42 through a suitableconstant threshold establishing device 54. Preferably, the thresholdestablishing device 54 comprises a zener diode which functionstocompensate for differences in electrical characteristics whendifferent switch devices are utilized. The current switch 50 maycomprise any one of a variety of current switch devices and in theillustrated embodiment is shown as a transistor having its baseconnected to the junction 42 through the zener diode 54. The transistor50 is illustrated as an NPN transistor although, if desired, the circuitmay be suitably modified to employ a PNP transistor. Thecollector-emitter circuit of the transistor 50 is coupled to the firstselectively energizable switch control means 48 which preferablycomprises a relay latch. coil adapted to effect opening of switch 10responsive to current flow therethrough, and to, retain switch 10 open.Accordingly, when the transistor 50. is rendered conductive, the coil 48which is serially connected to the collector-emitter circuit thereof isenergized. Furthermore, since latch coil 48 is operatively connected tothe switch 10, energization of the latch coil effects opening of theswitch. In addition, a diode 56 is connected across the latch coil 48 soas to provide transient protection for the coil in order to preventinadvertent energizationthereof due to spurious or transient signals.Thus, in operation, when the transistor 50 is rendered conductiveresponsive to, a variation in the impedance of the sensor 44, the latchcoil 48 is energized to effect opening of the switch 10, therebydisrupting power to the load 12. Furthermore, in accordance with animportant feature of the present invention the switch 10, the latch coil48 and the means 52 which comprises an unlatch coil, preferably define areed relay. Thus subsequent to energization of the latch coil 48, theswitch 10 is magnetically latched in its open position, i.e., the systemhas a memory function, whereby the switch 10 remains in an opencondition even if current flow through the coil 48 is halted untilpositive action is taken to temporarily neutralize the magnetic bias ofthe pennanent magnet (not shown) of the reed relay to permit the switch10 to return to its closed condition. Furthermore, this neutralizationmay be only accomplished if the fault condition has dissipated and theimpedance of the sensor 44 has returned to a level indicative of properoperation of the system being protected.

In order to effect removal of the previously established magnetic biasand permit the switch 10 to return to its closed position the unlatchedcoil 52 is connected in shunt with the latch coil 48 and is selectivelycoupled to the power supply network 16 through a normally open resetswitch 56 and through a current limiting resistor 58. The reset switch56 may comprise a manually actuatable push-button switch adapted tomomentarily connect the power supply 16 to the unlatch coil 52 so as topermit current to flow therethrough. In this connection the unlatch coil52 is coupled to the latch coil 48 and upon energization thereof amagnetic field is established across the un-. latch coil 52 of anopposite direction to the magnetic field established across the latchcoil 48 such that ener gization of the unlatch coil causesneutralization'of any previous magnetic bias, thereby permittingtheswitch 10 operatively connected to the latch coil 48 to returncomprises adiode having its anode connected to this junction and itscathode connected to the collector of transistor 50. The diode 62functions to maintain the unlatch coil 52 electrically isolated from thetransistor 50 when the transistor 50 is in a non-conductive conditionand also functionstoshunt current away from the unlatch coil 52 when thetransistor 50 is in a conductive condition so that closure of the resetswitch 56 is ineffective to cause neutralization of the magnetic bias,when the transistor 50 -is in aconductive condition. More particularly,it may be seen that when, the transistor 50 is conductive the diode 62is forward biased so that current cannot flow to the unlatch coil 52,thereby preventing energization of the unlatch coil 52. Thus, if thereset switch 56 is depressed; while the transistor 50 is in a conductivecondition responsive to a sensed fault condition the diode 62 is forwardbiased and current cannot flow through the coil 52, but instead isshunted away. from the coil 52 through the forward dias ed diode 62 andthrough the collector-emitter circuit of thetransistor 50; Similarly,the diode 62 func tions to block any current flow through the'latch coil48 to the unlatch coil 52. Similarly, when the transistor 50 isnonconductive the diode 62, is reverse biased and thus blocks currentflow therethrough so that upon closure of the reset switch 56 current ispositively'shunted away from the coil 48 which is hence prevented frombeing energize and instead current is caused to flow through the unlatchcoil 52 so that neutralization of the magnetic bias is effected; Thus,it may be seen that the provision of the diode 62 prevents energizationof the unlatch coil 52 until thefault condition is removed permittingthe transistor 50 to become non-conductive so that the diode 62 becomesreverse biased. Accordingly, it may be seenthat once a .fault conditionhas been sensed causing conduction off-transistor 50 and consequentenergization of the latch coil 48, energization of the unlatch coil 52to effect neutralization of the magnetic bias and reclosing of theswitch 10 cannot be effected until the fault condition has dissipatedand the impedance of the sensor 44 has returned to its proper level sothat transistor 50 becomes non-conductive, whereby closing of the resetswitch 56 is effective to permit energization of the unlatch coil 52. inaddition, in order to minimize contact errosion and sparking a suitablevoltage suppression system, such as a serially connected resistor 64 andcapacitor 66 is connected across the switch 10. n

Thus, a unique protection system has been provided and described indetail hereinabove in which the power being supplied to a load isdisrupted responsive to a sensed fault condition and the power isprevented from being reapplied to the load until dissipation of thesensed fault has occurred.

Various changes and modifications in the abovedescribed embodiment willbe readily apparent to those skilled in the art and any of such changesor modifications are deemed: to be within the spirit and scopeof thepresent invention as set forth in the appended claims. t

lclaim: v l.'A protection system for effecting disruption of the powerbeing supplied to a loadjresponsive to a sensed variation in theoperation .of a system being protected comprising;

a first selectively operable switch means for connectinganddisconnecting the load from a source of power in response to closingand opening thereof,

a first selectively energizable switch control means operativelyconnected to said first switch means adapted to effect opening of saidfirst switch means responsive to energization thereof, said first switchmeans maintained in its open condition until a neutralizing operation iseffected,

a selectively energizable current switch coupled to said switch controlmeans, said current switch effecting energization of said switch controlmeans responsive to conduction thereof,

means responsive to the operation of the system being protected forrendering said current switch conductive responsive to a preselectedvariation in the operation of the system being protected and selectivelyoperable neutralizing means coupled to said 'current switch and to saidswitch control means for effecting said neutralizing operation only whensaid current switch returns to a nonconductivecondition. 2. A protectionsystem in accordance with claim 1 wherein said means responsive to theoperation of the system being protected comprises a variable impedance,the impedance of which varies responsive to variations in tected. t

3. A protection system in accordance with claim 2- wherein said meansresponsive to the operation of the system being protected includes avoltage divider having a pair of legs defining a jun'ction, one of saidlegs being connected to said variable impedance, whereby first andsecond preselected voltage levels are established at said junctionresponsive. to first and second impedance valuesof said variableimpedance corresponding to desired and undesired operation respectivelyof the system being protected.

4. A system in accordance with claim 3 wherein said current switch iscoupled to said voltage divider junction and is adapted to be renderedconductive responsive to establishment of said second preselectedvoltage level at said junction, thereby energizing said switch controlmeans.

5. A system in accordance with claim 4 wherein said current switchcomprisesa transistor having its base coupled to said voltage dividerjunction and having its collector-emitter circuit serially coupled tosaid switch control means, said collector-emitter circuit defining a lowimpedance current path upon conduction of said transistor whilesubstantially precluding current flow therethrough upon non-conductionof said transistor.

6. A system in accordance with claim 4 wherein said switch controlincludes a first relay coil coupled to said current switch, said firstrelay coil being energized responsive to current flow through saidcurrent switch for rendering said first switch means non-conductive theoperation'of the system being proupon the initiation of current flowthrough said first relay coil causing magnetic retention of said firstswitch means in a non-conductive condition.

7. A system in accordance with claim 6 wherein said neutralizing meansincludes a second selectively energizable relay coil adapted toselectively neutralize the magnetic retention of said first switch meansin a nonconductive condition responsive to current flow through saidsecond relay coil.

8. A system in accordance with claim 7 wherein said first relaycoilcomprises a latch coil serially connected to said current switch, saidlatch coil effecting opening and retaining in an open condition of saidfirst switch means until current flow is established through said secondrelay coil.

9. A system in accordance with claim 8 wherein said second relay coilcomprises an unlatch coil connected in shunt relationship with saidlatch coil.

10. A system in accordance with claim ,9 wherein means are provided forelectrically isolating said unlatch coil from the source of power toprevent current flow therethrough when said current switch is in aconductive state.

11. A system in accordance with claim 10 wherein said. means forelectrically isolating said unlatch coil from the source of powercomprises a unidirectional switch connected between said unlatch coiland said current switch, said unidirectional switch being renderedconductive for shunting current flow from said unlatch coil through saidcurrent switch when said current switch is in a conductive state.

12. A system in accordance with claim 11 wherein said unidirectionalswitch comprises a diode.

l3. Asystem in accordance with claim 11 wherein a selectively operablereset switch is coupled between the source of power and said unlatchcoil'for effecting current flow through said unlatch coil upon closurethereof only when said current switch remains in a nonconductivecondition.

-l4. A protection system for disrupting the power to a load responsiveto a sensed fault condition and precluding the re-application of powerto the load prior to dissipation of the fault condition comprising;

a first normally closed switch means for connecting the source of powerto the load,

a relay including a latch coil operatively connected to the switch meansfor effecting opening thereof 8 upon energization of said latch coil,said latch coil permitting a magnetic bias to effect retention of saidswitch means in an open condition subsequent to de-energization thereof,and an unlatch coil for efiecting neutralization of the magnetic biasupon energization of said unlatch coil, normally non-conductivesemi-conductor switch device having a voltage responsive control elementfor controlling conduction thereof serially connected to said latch coiland connected in shunt across said unlatch coil forefiecting'energization of said latch coil and preventing energization ofsaid unlatch coil upon conduction thereof, sensing means coupled to saidcontrol element for controlling conduction of said semi-conductorswitch, said sensing means including a variable impedance coupled to thepower source whereby a variation in its impedance responsive to a sensedfault condition vanes the voltage established thereacross so as torender said semi-conductor switch device conductive, .a selectivelyoperable reset switch coupled between the power source and said unlatchcoil for effecting energization thereof upon closure of said resetswitch, said reset switch being connected in shunt with said latch coilfor positively shunting current from said latch coil only when saidsemi-conductor current switch device is rendered non-conductive upondissipation of the fault condition, and

shunt switch means coupled between said unlatch coil and said resetswitch, said shunt switch means being rendered conductive responsive toconduction of said semi-conductor switch for shunting current away fromsaid unlatch coil until said semiconductor switch is renderednon-conductive whereupon current flow through said unlatch coil ispermitted.

15. A system in accordance with claim 14 wherein said shunt switch meanscomprises a diode having one tenninal connected intermediate said resetswitch and said unlatch coil and its other terminal coupled to saidsemi-conductor switch.

16. A system in accordance with claim 15 wherein said semi-conductorswitch comprises a transistor having its base coupled to said sensingmeans and its collector-emitter circuit serially connected to said latchcoil and in shunt across said unlatch coil.

1. A protection system for effecting disruption of the power beingsupplied to a load responsive to a sensed variation in the operation ofa system being protected comprising; a first selectively operable switchmeans for connecting and disconnecting the load from a source of powerin response to closing and opening thereof, a first selectivelyenergizable switch control means operatively connected to said firstswitch means adapted to effect opening of said first switch meansresponsive to energization thereof, said first switch means maintainedin its open condition until a neutralizing operation is effected, aselectively energizable current switch coupled to said switch controlmeans, said current switch effecting energization of saiD switch controlmeans responsive to conduction thereof, means responsive to theoperation of the system being protected for rendering said currentswitch conductive responsive to a preselected variation in the operationof the system being protected, and selectively operable neutralizingmeans coupled to said current switch and to said switch control meansfor effecting said neutralizing operation only when said current switchreturns to a non-conductive condition.
 2. A protection system inaccordance with claim 1 wherein said means responsive to the operationof the system being protected comprises a variable impedance, theimpedance of which varies responsive to variations in the operation ofthe system being protected.
 3. A protection system in accordance withclaim 2 wherein said means responsive to the operation of the systembeing protected includes a voltage divider having a pair of legsdefining a junction, one of said legs being connected to said variableimpedance, whereby first and second preselected voltage levels areestablished at said junction responsive to first and second impedancevalues of said variable impedance corresponding to desired and undesiredoperation respectively of the system being protected.
 4. A system inaccordance with claim 3 wherein said current switch is coupled to saidvoltage divider junction and is adapted to be rendered conductiveresponsive to establishment of said second preselected voltage level atsaid junction, thereby energizing said switch control means.
 5. A systemin accordance with claim 4 wherein said current switch comprises atransistor having its base coupled to said voltage divider junction andhaving its collector-emitter circuit serially coupled to said switchcontrol means, said collector-emitter circuit defining a low impedancecurrent path upon conduction of said transistor while substantiallyprecluding current flow therethrough upon non-conduction of saidtransistor.
 6. A system in accordance with claim 4 wherein said switchcontrol includes a first relay coil coupled to said current switch, saidfirst relay coil being energized responsive to current flow through saidcurrent switch for rendering said first switch means non-conductive uponthe initiation of current flow through said first relay coil causingmagnetic retention of said first switch means in a non-conductivecondition.
 7. A system in accordance with claim 6 wherein saidneutralizing means includes a second selectively energizable relay coiladapted to selectively neutralize the magnetic retention of said firstswitch means in a non-conductive condition responsive to current flowthrough said second relay coil.
 8. A system in accordance with claim 7wherein said first relay coil comprises a latch coil serially connectedto said current switch, said latch coil effecting opening and retainingin an open condition of said first switch means until current flow isestablished through said second relay coil.
 9. A system in accordancewith claim 8 wherein said second relay coil comprises an unlatch coilconnected in shunt relationship with said latch coil.
 10. A system inaccordance with claim 9 wherein means are provided for electricallyisolating said unlatch coil from the source of power to prevent currentflow therethrough when said current switch is in a conductive state. 11.A system in accordance with claim 10 wherein said means for electricallyisolating said unlatch coil from the source of power comprises aunidirectional switch connected between said unlatch coil and saidcurrent switch, said unidirectional switch being rendered conductive forshunting current flow from said unlatch coil through said current switchwhen said current switch is in a conductive state.
 12. A system inaccordance with claim 11 wherein said unidirectional switch comprises adiode.
 13. A system in accordance with claim 11 wherein a selectivelyoperable reset switch is coupled between the source of power and saidunlAtch coil for effecting current flow through said unlatch coil uponclosure thereof only when said current switch remains in anon-conductive condition.
 14. A protection system for disrupting thepower to a load responsive to a sensed fault condition and precludingthe re-application of power to the load prior to dissipation of thefault condition comprising; a first normally closed switch means forconnecting the source of power to the load, a relay including a latchcoil operatively connected to the switch means for effecting openingthereof upon energization of said latch coil, said latch coil permittinga magnetic bias to effect retention of said switch means in an opencondition subsequent to de-energization thereof, and an unlatch coil foreffecting neutralization of the magnetic bias upon energization of saidunlatch coil, a normally non-conductive semi-conductor switch devicehaving a voltage responsive control element for controlling conductionthereof serially connected to said latch coil and connected in shuntacross said unlatch coil for effecting energization of said latch coiland preventing energization of said unlatch coil upon conductionthereof, sensing means coupled to said control element for controllingconduction of said semi-conductor switch, said sensing means including avariable impedance coupled to the power source whereby a variation inits impedance responsive to a sensed fault condition varies the voltageestablished thereacross so as to render said semi-conductor switchdevice conductive, a selectively operable reset switch coupled betweenthe power source and said unlatch coil for effecting energizationthereof upon closure of said reset switch, said reset switch beingconnected in shunt with said latch coil for positively shunting currentfrom said latch coil only when said semi-conductor current switch deviceis rendered non-conductive upon dissipation of the fault condition, andshunt switch means coupled between said unlatch coil and said resetswitch, said shunt switch means being rendered conductive responsive toconduction of said semi-conductor switch for shunting current away fromsaid unlatch coil until said semi-conductor switch is renderednon-conductive whereupon current flow through said unlatch coil ispermitted.
 15. A system in accordance with claim 14 wherein said shuntswitch means comprises a diode having one terminal connectedintermediate said reset switch and said unlatch coil and its otherterminal coupled to said semi-conductor switch.
 16. A system inaccordance with claim 15 wherein said semi-conductor switch comprises atransistor having its base coupled to said sensing means and itscollector-emitter circuit serially connected to said latch coil and inshunt across said unlatch coil.